Photoresist compositions are used in microlithography processes for making miniaturized electronic components such as in the fabrication of computer chips and integrated circuits. Generally, in these processes, a thin coating of a film of a photoresist composition is first applied to a substrate material, such as silicon wafers used for making integrated circuits. The coated substrate is then baked to evaporate any solvent in the photoresist composition and to fix the coating onto the substrate. The baked and coated surface of the substrate is next subjected to an image-wise exposure to radiation.
This radiation exposure causes a chemical transformation in the exposed areas of the coated surface. Visible light, ultraviolet (UV) light, electron beam and X-ray radiant energy are radiation types commonly used today in microlithographic processes. After this image-wise exposure, the coated substrate is treated with a developer solution to dissolve and remove either the radiation-exposed or the unexposed areas of the photoresist.
There are two types of photoresist compositions, negative-working and positive-working. When positive-working photoresist compositions are exposed image-wise to radiation, the areas of the photoresist composition exposed to the radiation become soluble in a developer solution (e.g. a rearrangement reaction occurs) while the unexposed areas of the photoresist coating remain relatively insoluble to such a solution. Thus, treatment of an exposed positive-working photoresist with a developer causes removal of the exposed areas of the photoresist coating and the formation of a positive image in the coating, thereby uncovering a desired portion of the underlying substrate surface on which the photoresist composition was deposited. In a negative-working photoresist the developer removes the portions that are not exposed.
The trend towards the miniaturization of semiconductor devices has led both to the use of new photoresists that are sensitive to lower and lower wavelengths of radiation, and also to the use of sophisticated multilevel systems to overcome difficulties associated with such miniaturization.
High resolution, chemically amplified, deep ultraviolet (100-300 nm) positive and negative tone photoresists are available for patterning images with less than quarter micron geometries. There are two major deep ultraviolet (uv) exposure technologies that have provided significant advancement in miniaturization, and these are lasers that emit radiation at 248 nm and 193 nm. Examples of such photoresists are given in the following patents and incorporated herein by reference, U.S. Pat. Nos. 4,491,628, 5,350,660, EP 794458 and GB 2320718. Photoresists for 248 nm have typically been based on substituted polyhydroxystyrene and its copolymers. On the other hand, photoresists for 193 nm exposure require non-aromatic polymers, since aromatics are opaque at this wavelength. Generally, alicyclic hydrocarbons are incorporated into the polymer to replace the etch resistance lost by eliminating the aromatic functionality. Furthermore, at lower wavelengths the reflection from the substrate becomes increasingly detrimental to the lithographic performance of the photoresist. Therefore, at these wavelengths antireflective coatings become critical.
The use of highly absorbing antireflective coatings in photolithography is a simpler approach to diminish the problems that result from back reflection of light from highly reflective substrates. Two major disadvantages of back reflectivity are thin film interference effects and reflective notching. Thin film interference can cause swing effects that result in changes in critical line width dimensions caused by variations in the total light intensity in the resist film as the thickness of the resist changes and standing waves that result in wavy feature edges stemming from dose oscillating in the vertical direction. Reflective notching becomes severe as the photoresist is patterned over substrates containing topographical features, which scatter light through the photoresist film, leading to line width variations, and in the extreme case, forming regions with complete photoresist loss (for positive resists) or unexpected photoresist masking (for negative resists).
The use of bottom antireflective coatings provides the best solution for the elimination of reflectivity. The bottom antireflective coating is applied on the substrate and then a layer of photoresist is applied on top of the antireflective coating. The photoresist is exposed imagewise and developed. The antireflective coating in the exposed area of a positive photoresist is then typically etched and the photoresist pattern is thus transferred to the substrate.
A consequence of using an antireflective coating is its effect on etch rate selectivity as compared to the photoresist that is coated over the antireflective coating. In most single layer pattern transfer processes, an important and desired property of antireflective coatings is their high etch rates in plasmas. It is well known in the semiconductor industry that a antireflective coating that has a significantly higher etch rate than the photoresist will be better in successfully transfer the pattern after exposure and further processing steps. This, however, makes it difficult to formulate both antireflective coatings and photoresists since both materials are based on similar types of polymers. While one way to control the etch property of the antireflective coating is by the selection of the polymer dye used in the antireflective coating material, this can lead to formulation issues in conjunction with its use with photoresists. Thus, there is a need to develop an antireflective coating composition that has good etch rate and etch rate selectivity that is not dependent upon the polymers used in the antireflective coatings and photoresist.
Etch selectivity is a measure of etch rate removal of one material compared to another. Often, the resist and the antireflective coating are compositionally and structurally similar, which leads to a lack of selectivity between these two materials even if the condition under which the antireflective coating break-through steps are performed are changed, for example, altering plate bake temperature, enchant gases, voltage biases, pressures, and the like.
There are several approaches to creating etch selectivity differences between the resist and antireflective coating. For example, by making the antireflective coating compositionally and structurally different from the resist (by, for example, incorporating as much oxygen content into the antireflective coating resins), the obtained selectivity difference is more or less constant when changing etch process conditions as mentioned above.
It is well known in the semiconductor industry that an antireflective coating that has a significantly higher etch rate than the photoresist will be better in successfully transfer the pattern after exposure and further processing steps. There are also applications where a matched etch rate would be desirable (no selectivity) as in the case of gate trimming or in via filling where etch times can be reduced by selecting conditions amenable to higher etch rates during the via fill removal step. Thus, there is a need to develop an antireflective coating composition that has etch rate selectivity that can be tuned to facilitate both pattern transfer and CD trimming or in schemes where antireflective coatings are required to be have different selectivities at different steps in pattern transfer process.
The inventors have found that polymers that have a ceiling temperature (the temperature at which polymerization and monomer formation are at equilibrium) at or near the etch plate temperature in the etch chamber, the etch rate selectivity of the antireflective coating can be tuned, depending upon the requirements of the semiconductor engineer.